Synthetic reinforced interbody fusion implants

ABSTRACT

Interbody fusion implants that include a load bearing body composed of a calcium phosphate material hardened around one or more structural reinforcing members are provided. The reinforcing members aid the load bearing body in resisting bending forces and, in certain forms of the invention, aid in preventing expulsion of the implant after implantation. Methods for promoting fusion bone growth in the space between adjacent vertebrae and methods for making the inventive implants are also provided.

BACKGROUND OF THE INVENTION

[0001] The present invention broadly concerns medical implants. Morespecifically, the invention provides reinforced interbody fusionimplants and methods for making and using the implants.

[0002] Intervertebral discs, located between the endplates of adjacentvertebrae, stabilize the spine, distribute forces between vertebrae andcushion vertebral bodies. A normal intervertebral disc includes asemi-gelatinous component, the nucleus pulposus, which is surrounded andconfined by an outer, fibrous ring called the annulus fibrosus. In ahealthy, undamaged spine, the annulus fibrosus prevents the nucleuspulposus from protruding outside the disc space.

[0003] Spinal discs may be displaced or damaged due to trauma, diseaseor aging. Disruption of the annulus fibrosus allows the nucleus pulposusto protrude into the vertebral canal, a condition commonly referred toas a herniated or ruptured disc. The extruded nucleus pulposus may presson a spinal nerve, which may result in nerve damage, pain, numbness,muscle weakness and paralysis. Intervertebral discs may also deterioratedue to the normal aging process or disease. As a disc dehydrates andhardens, the disc space height will be reduced leading to instability ofthe spine, decreased mobility and pain.

[0004] Sometimes the only relief from the symptoms of these conditionsis a discectomy, or surgical removal of a portion or all of anintervertebral disc followed by fusion of the adjacent vertebrae. Theremoval of the damaged or unhealthy disc will allow the disc space tocollapse. Collapse of the disc space can cause instability of the spine,abnormal joint mechanics, premature development of arthritis or nervedamage, in addition to severe pain. Pain relief via discectomy andarthrodesis requires preservation of the disc space and eventual fusionof the affected motion segments.

[0005] Bone grafts are often used to fill the intervertebral space toprevent disc space collapse and promote fusion of the adjacent vertebraeacross the disc space. In early techniques, bone material was simplydisposed between the adjacent vertebrae, typically at the posterioraspect of the vertebra, and the spinal column was stabilized by way of aplate or rod spanning the affected vertebrae. Once fusion occurred, thehardware used to maintain the stability of the segment becamesuperfluous and was a permanent foreign body. Moreover, the surgicalprocedures necessary to implant a rod or plate to stabilize the levelduring fusion were frequently lengthy and involved.

[0006] It was therefore determined that a more optimal solution to thestabilization of an excised disc space is to fuse the vertebrae betweentheir respective end plates, preferably without the need for anterior orposterior plating. There have been an extensive number of attempts todevelop an acceptable intradiscal implant that could be used to replacea damaged disc and maintain the stability of the disc interspace betweenthe adjacent vertebrae, at least until complete arthrodesis is achieved.The implant must provide temporary support and allow bone ingrowth.Success of the discectomy and fusion procedure requires the developmentof a contiguous growth of bone to create a solid mass because theimplant may not withstand the compressive loads on the spine for thelife of the patient.

[0007] There is a continuing need for interbody fusion implants whichhave sufficient strength to support the vertebral column until after theadjacent vertebrae are fused and which eliminate or at least minimizeany permanent foreign body after the fusion. SUMMARY OF THE INVENTION

[0008] In accordance with one aspect of the present invention, animplant includes a porous, biocompatible load bearing body composed of asynthetic calcium phosphate material that is hardened around at leastone structural reinforcing member. The reinforcing member advantageouslyhelps the load bearing body resist bending forces when implanted. Thebody is typically sized and configured for engagement between twovertebrae and has a superior surface configured to contact onevertebrae, and an inferior surface configured to contact anothervertebrae. The reinforcing member is preferably an internal member andis disposed between the superior surface and inferior surface, extendingalong a length of the body.

[0009] In yet other embodiments, the implant includes a load bearingbody composed of a hardened synthetic calcium phosphate material and atleast one structural reinforcing member for resisting expulsion afterimplantation. The structural reinforcing member is at least partiallyembedded in the load bearing body and configured to contact adjacentvertebrae. The body is sized and configured for engagement between twovertebrae and has a superior surface and an inferior surface.

[0010] In yet another aspect of the invention, methods of promotingfusion bone growth between adjacent vertebrae are provided. In one formof the invention, a method includes providing an interbody fusionimplant described above, preparing an adjacent vertebrae to receive theimplant in an intervertebral space between adjacent vertebrae andplacing the implant into the intervertebral space.

[0011] Other aspects of the invention provide methods for making theinterbody fusion implants of the present invention. The preferredmethods include providing a mold having positioned therein a structuralreinforcing member, passing a hardenable synthetic calcium phosphatematerial into the mold, and causing the material to harden to form aload bearing implant.

[0012] These and other objects and advantages of the present inventionwill be apparent from the descriptions herein.

BRIEF DESCRIPTION OF THE FIGURES

[0013]FIG. 1 depicts a perspective view of one embodiment of aninterbody fusion implant.

[0014]FIG. 2 depicts an end view of the implant of FIG. 1.

[0015]FIG. 3 depicts a perspective view of an alternative embodiment ofthe interbody fusion implant of the present invention.

[0016]FIG. 4 depicts an end view of the implant of FIG. 3.

[0017]FIG. 5 depicts a perspective view of a structural reinforcingmember used to reinforce the implant of FIG. 1.

[0018]FIG. 6 depicts a perspective view of an alternative embodiment ofa reinforcing member.

[0019]FIG. 7 depicts an end view of the reinforcing member of FIG. 7.

[0020]FIG. 8 depicts a perspective view of an alternative embodiment ofa reinforcing member.

[0021]FIG. 9 depicts a perspective view of an alternative embodiment ofa reinforcing member.

[0022]FIG. 10 depicts an end view of the reinforcing member of FIG. 9.

[0023]FIG. 11 depicts a perspective view of an alternative embodiment ofa reinforcing member.

[0024]FIG. 12 depicts a perspective view of a helical-shaped reinforcingmember.

[0025]FIG. 13 depicts a perspective view of an alternative embodiment ofa reinforcing member.

[0026]FIG. 14 depicts an end view of the reinforcing member of FIG. 13.

[0027]FIG. 15 depicts a perspective view of an alternative embodiment ofa reinforcing member.

[0028]FIG. 16 depicts an end view of the reinforcing member of FIG. 15.

[0029]FIG. 17 depicts a perspective view of an alternative embodiment ofa reinforcing member.

[0030]FIG. 18 depicts an end view of the reinforcing member of FIG. 17.

[0031]FIG. 19 depicts a perspective view of an alternative embodiment ofa reinforcing member.

[0032]FIG. 20 depicts an end view of the reinforcing member of FIG. 19.

[0033]FIG. 21 depicts a perspective view of a wedge-shaped interbodyfusion implant.

[0034]FIG. 22 depicts an end view of the implant of FIG. 21.

[0035]FIG. 23 depicts a reinforcing member that may reinforce thewedge-shaped implant of FIG. 25.

[0036]FIG. 24 depicts a perspective view of an elliptical-shapedinterbody fusion implant.

[0037]FIG. 25 depicts a perspective view of the reinforcing member ofthe implant of FIG. 24.

[0038]FIG. 26 depicts a perspective view of an alternative embodiment ofthe interbody fusion implant of the present invention, having a scoremark in one end.

[0039]FIG. 27 depicts a perspective view of an alternative embodiment ofan interbody fusion implant of the present invention, showing a loadbearing body reinforced with a spiral reinforcing member that formsthreads on the outer surface of the implant.

[0040]FIG. 28 depicts an end view of the implant of FIG. 27.

[0041]FIG. 29 depicts a perspective view of an alternative embodiment ofan interbody fusion implant of the present invention.

[0042]FIG. 30 depicts an end view of the implant of FIG. 29.

[0043]FIG. 31 depicts a perspective view of reinforcing member 270, withradially extending plates 260, that may be used to reinforce the implantof FIG. 30.

[0044]FIG. 32 depicts a top view of two implants of the presentinvention bilaterally implanted within an intervertebral space.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to preferred embodimentsand specific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications of the invention, and such further applications of theprinciples of the invention as illustrated herein, being contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

[0046] As disclosed above, the present invention relates generally tosynthetic reinforced medical implants. One specific aspect of theinvention provides interbody fusion implants that include a porous,biocompatible load bearing body formed of a synthetic calcium phosphatematerial hardened around at least one internal reinforcing member forresisting bending or tensile forces when implanted. The implant mayinclude a low crystallinity calcium phosphate material that isself-hardening, and requires no externally applied heat or pressure toharden, formed around a metallic reinforcing member, such as a metallicmesh. In alternative embodiments, the material is hardenable uponexposure to pressure and/or a temperature of about 5° C. to about 50°C., typically about 20° C. to 40° C.

[0047] Such implants are advantageous, for example, in minimizing themetal artifact in computer tomography (CT) or magnetic resonance imaging(MRI) which makes post-operative complications diagnosis easier. It isalso easier to assess the fusion radiographically. Moreover, the abovecalcium phosphate materials may degrade over time and be replaced bybone. In addition, such implants may be constructed to provide for therelative absence of stress shielding, and make it easier to assess thefusion after the ceramic has degraded. Additionally, direct boneapposition to the calcium phosphate instead of possible fibrous tissueinterfaces with metal devices is advantageous.

[0048] Referring now to FIGS. 1-5, an implant 10 may include a loadbearing body 20 having disposed therein structural reinforcing members30. Load bearing body 20 has a first end 21, a second end 22, and a wall23 connecting first end 21 and second end 22. Wall 23 defines a first,superior surface 24 and a second, inferior surface 25 that areconfigured to contact adjacent vertebrae. Load bearing body 20 mayoptionally include a thru-hole 26 that may be filled with osteogenicmaterial as further described below. Other configurations will beapparent to the skilled artisan. For example, in other embodiments, thehole may not extend completely through the load bearing body. The loadbearing body may define a cavity, or other discontinuity on the superiorand/or inferior surface that may also be advantageously filled withosteogenic material. The load bearing body may further include atool-engagement end 27 that defines a tool engaging, or instrumentattachment hole 28 as seen in FIGS. 3 and 4, wherein body 20′ alsoincludes a second end 22′, a wall 23′, a superior surface 24′ and aninferior surface 25′. The body may further include a score 29, as seenin FIGS. 3 and 4, for indicating the orientation of other components ofthe implant 10, for example the hole 26 and/or the reinforcing members30, as well as external threads 190.

[0049] Load bearing body 20 is preferably formed of a hardenable calciumphosphate material. A wide variety of calcium phosphate materials may beused including hydroxyapatite, tricalcium phosphate and mixturesthereof. The calcium phosphate material of which the load bearing bodyis composed preferably has a composition substantially similar tonatural bone. Furthermore, a preferred synthetic calcium phosphatematerial is one that is flowable at a low temperature, such as belowabout 50° C., especially room temperature (about 25° C.), and ishardenable at such temperatures. More preferred materials will beflowable at room temperature (about 25° C.) and hardenable at about bodytemperature (about 37° C.). Such synthetic calcium phosphate materialsinclude a poorly or low crystalline calcium phosphate, such as a low orpoorly crystalline apatite, including hydroxyapatite, available fromEtex Corporation and as described in U.S. Pat. Nos. 5,783,217;5,676,976; 5,683,461; and 5,650,176, and PCT International PublicationNos. WO 98/16268, WO 96/39202 and WO 98/16209, all to Lee et al. Asdefined in the recited patents and herein, by “poorly or lowcrystalline” calcium phosphate material is meant a material that isamorphous, having little or no long range order and/or a material thatis nanocrystalline exhibiting crystalline domains on the order ofnanometers or Angstroms. The calcium:phosphate ratio of the load bearingbody is typically in the range of about 1.3 to 1.7, more typically about1.5 to 1.7.

[0050] Other additives may be included in the compositions that form theload bearing bodies of the present invention to adjust their properties,including supporting or strengthening filler materials, pore formingagents and osteoinductive factors as described below.

[0051] As discussed above, and as seen in FIGS. 1-5, implant 10 includesat least one structural reinforcing member 30 disposed therein. FIGS.1-4 depict two structural reinforcing members 30 disposed along thelength of implant 10. The calcium phosphate material is preferablyhardened around structural reinforcing members 30, such that thereinforcing members are contained within the load bearing body. Althoughthe members shown in FIGS. 1-4 are completely surrounded by the loadbearing body, they may, in alternative embodiments, be partiallyexposed.

[0052] The reinforcing members may be disposed between the superiorsurface and the inferior surface, and may extend along a length, of theload bearing body, including extending non-parallel, such as obliquelyor transverse, or parallel to the superior and inferior surfaces of thebody. Additionally, the reinforcing members may extend non-parallel,including obliquely or transverse, and in other forms may extendparallel, to the central longitudinal axis of the load bearing body ofthe implants.

[0053] Structural reinforcing members 30 may assume a wide variety ofshapes. For example, member 30 may be cylindrical-shaped as best seen inFIG. 5. Member 30 may assume other shapes known in the art, includingspherical, pyramidal, rectangular and other polygonal shapes. FIGS. 6-16depict a variety of other ways in which the structural members may beconfigured.

[0054]FIGS. 6 and 7 depict a structural member 50 having twolongitudinal members 51 connected at either end 52 to end members 53.Each end 52 of longitudinal members 51 is attached to an internalsurface 54 of end members 53. Longitudinal members 51 are preferablyelongated members, such as cylindrical-shaped members, and are furtherpreferably positioned generally parallel to each other. Longitudinalmembers 51 and end members 53 may be constructed independently and thenjoined by methods known to the art, or may be made as a single, integralunit or by any other variation known to the skilled artisan.

[0055] Referring now to FIG. 8, a reinforcing band member 60 is shownthat is preferably a length of material that has been formed into asubstantially ovate shape with rounded ends 61, sides 63 and defining agap 62. The dimensions of reinforcing member 60, as with all thereinforcing members described herein, are such that will reinforce theload bearing body against bending forces. Such bending forces imposedupon the implant in situ include, for example, tensile forces andcompressive forces. Referring now to FIG. 9, a reinforcing member 70 mayinclude a reinforcing band 71 and attached intermediate members 72 toform reinforcing member 70. Intermediate members 72 span the area, orgap 73 defined by reinforcing band 71 to form reinforcing member 70, andare positioned preferably such that a longitudinal axis AG of theintermediate members 72 is perpendicular to the longitudinal axis A_(R)of reinforcing member 71 as seen in FIGS. 9 and 10. Two intermediatemembers 72 are seen in FIG. 9, although less than or more than thisnumber may be present in order to affect the structural integrity of theload bearing body into which it is incorporated. Intermediate members 72shown in FIGS. 9 and 10 are ring-shaped structures, but may also assumeother shapes known in the art as described above.

[0056] Referring now to FIG. 11, reinforcing member 80 is shown.Reinforcing member 80 is an elongated plate 81 having an outer surface82. Plate 81 may further include thru-holes 83 disposed along the lengthof plate 81. One or more of reinforcing member 80 may be disposed withina particular load bearing body.

[0057] Referring now to FIG. 12, a reinforcing member 90 shaped in aspiral, or helical configuration is shown. This particular configurationis advantageous in that, in one preferred form of the invention,reinforcing member 90 may be only partially embedded in the load bearingbody such that it forms threads on the outer surface of the body as morefully described below.

[0058] The reinforcing members, or scaffolds, described herein may bemade of a wide variety of materials that resist bending or tensileloads. Such materials will therefore increase the structural integrityof the load bearing bodies described herein. The reinforcing member ispreferably formed of a metallic material, including titanium, stainlesssteel, tantalum and alloys thereof, as well as cobalt-chromium,cobalt-chromium-nickel and cobalt-chromium-molybdenum alloys. Thereinforcing member may also be formed of other materials, for example,carbon fiber, carbon fiber composites, collagen strands (e.g. fibers orwoven ropes), or plastics such as polyethylene, Dacron®, and degradablepolymers. The reinforcing member will advantageously be combined withthe load-bearing body to form an implant able to withstand compressiveforces of at least about 40 MPa.

[0059] In one preferred embodiment of the present invention, thereinforcing members described above are composed of a mesh, such as atitanium mesh. The mesh may be formed into a reinforcing member thatwill form the shaped members described above. For example, metallic meshmay be shaped into several configurations that will form thecylindrical-shaped reinforcing members described in FIGS. 5 and 6. Stillfurther alternative reinforcing members are shown in FIGS. 17-24,discussed more fully below.

[0060] Referring now to FIGS. 13 and 14, reinforcing member 100 includesthree generally ovate rings 101, 102 and 103 which are attached at theirends to form an overall, generally cylindrical shape. Reinforcing member100 also includes end rings 104 and 105 to which ovate rings 101, 102and 103 are attached at points of intersection to provide additionalstability to the reinforcement member 100.

[0061] Referring now to FIGS. 15 and 16, reinforcing member 110 is shownand is identical to reinforcing member 100 except for the presence ofattached intermediate rings 111 and 112 along the length of the ovaterings to provide still further stability.

[0062] Referring now to FIGS. 17 and 18, reinforcing member 120 includesa central wire 121 a having an ovate shape with rounded ends 122, sides124 a and 124 b and defining a gap, or area 123. Three additional ovatewire members, identified as upper wire 121 b, medial wire 121 c andlower wire 121 d are disposed along the length of central wire 121 a,and may be positioned one on top of each other, between sides 124 a and124 b, such that a longitudinal plane passing independently through ofeach of the upper, medial and lower wires is non-parallel, e.g.perpendicular, to a similar longitudinal plane passing through centralwire 121 a, although other configurations are also envisioned.Reinforcing member 120 further includes wire stabilizer rings 125connected to wires 121 a 121 d at intersecting locations.

[0063] Referring now to FIGS. 19 and 20, reinforcing member 130 is shownthat includes upper, medial and lower wires 131 a, 131 b and 131 c,respectively, disposed in the same configuration as shown forreinforcing member 120. Reinforcing member 130 includes end stabilizers135 disposed about and connected at intersecting points to wires 131a-131 c. End stabilizers 135 are preferably formed from a ring-shapedwire that is bent such that the profile of the ring-shaped wire isarcuate as best seen in the end view of reinforcing member 130 shown inFIG. 20.

[0064] Referring now to FIGS. 21 and 22, implant 40 includes a loadbearing body 41 that is substantially rectangular in shape, and includesa first, superior surface 42, a second, inferior surface 43, and a wall44 connecting the two surfaces. Wall 44 is preferably of a heightapproximating that of an intervertebral disc space of a mammal, such asa human. Load bearing body 41 may further define a thru-hole 45 intowhich osteogenic material may be disposed. In alternative embodiments,load bearing body may define a cavity or other discontinuity on superiorsurface 42 and/or inferior surface 43 into which osteogenic material maybe disposed. Reinforcing member 140 is disposed within load bearing body40.

[0065] Reinforcing member 140 for load bearing body 40 is depictedseparately in FIG. 23. Reinforcing member 140 includes a body 141 thatis also substantially rectangular-shaped and defines a gap 143 toprovide an opening corresponding to the location of thru-hole 45 ofimplant 40 (see FIG. 21).

[0066] Yet another example of an implant configuration of the presentinvention is shown in FIGS. 24 and 25. Implant 150 is shown includingload bearing body 151 that is substantially elliptical in shape, andincludes a first, superior surface 152, a second, inferior surface 153and a wall 154 connecting first surface 152 and second surface 153. Loadbearing body 151 may also include a thru-hole 155 or other area whichmay be used for containing osteogenic material therein as discussedabove. Internal structural reinforcing member 160 is disposed withinload bearing body 151. As seen in FIG. 25, reinforcing member 160includes a body 161 that is substantially elliptical in shape anddefines a gap 163 to provide an opening corresponding to the location ofthru-hole 155 of implant 150.

[0067] As mentioned above, the thru-holes or other apertures ordiscontinuities may be filled with an osteogenic material. Any suitableosteogenic material or composition is contemplated, including autograft,allograft, xenograft, demineralized bone, synthetic and natural bonegraft substitutes, such as bioceramics, polymers, and osteoinductivefactors. The terms osteogenic material or osteogenic composition as usedherein mean virtually any material that promotes bone growth or healingincluding autograft, allograft, xenograft, bone graft substitutes andnatural, synthetic and recombinant proteins, nucleotide sequences (e.g.genes such as growth factor genes), hormones and the like.

[0068] Autograft can be harvested from locations such as the iliac crestusing drills, gouges, curettes, trephines and other tools and methodswhich are well known to surgeons in this field. Preferably, autograft isharvested from the iliac crest with minimally invasive surgery. Theosteogenic material may also include bone reamed away by the surgeonwhile preparing the end plates for the implant.

[0069] Advantageously, where autograft is chosen as the osteogenicmaterial, only a very small amount of bone material is needed to packthe thru-hole. The autograft itself is not required to providestructural support as this is provided by the implant. The donor surgeryfor such a small amount of bone is less invasive and better tolerated bythe patient. There is usually little need for muscle dissection inobtaining such small amounts of bone. The present invention thereforeeliminates or minimizes many of the disadvantages of employingautograft.

[0070] Natural and synthetic graft substitutes which replace thestructure or function of bone are also contemplated for the osteogeniccomposition. Any such graft substitute is contemplated, including forexample, demineralized bone matrix, mineral compositions andbioceramics. As is evident from a review of An Introduction toBioceramics, edited by Larry L. Hench and June Wilson (World ScientificPublishing Co. Ptd. Ltd, 1993, volume 1), there is a vast array ofbioceramic materials, including BIOGLASS®, hydroxyapatite and calciumphosphate compositions known in the art which can be used to advantagefor this purpose. This disclosure is herein incorporated by referencefor this purpose. Preferred compositions include bioactive glasses,tricalcium phosphates and hydroxyapatites. In one embodiment, the graftsubstitute is a biphasic calcium phosphate ceramic including tricalciumphosphate and hydroxyapatite.

[0071] In some embodiments, the osteogenic compositions used in thisinvention comprise a therapeutically effective amount to stimulate orinduce bone growth of a substantially pure bone inductive or growthfactor or protein in a pharmaceutically acceptable carrier. Thepreferred osteoinductive factors are the recombinant human bonemorphogenetic proteins (rhBMPs) because they are available in unlimitedsupply and do not transmit infectious diseases. Most preferably, thebone morphogenetic protein is a rhBMP-2, rhBMP-4, rhBMP-7, orheterodimers thereof.

[0072] Recombinant BMP-2 can be used at a concentration of about 0.4mg/ml to about 1.5 mg/ml, preferably near 1.5 mg/ml. However, any bonemorphogenetic protein is contemplated including bone morphogeneticproteins designated as BMP-1 through BMP-18. BMPs are available fromGenetics Institute, Inc., Cambridge, Mass. and the BMPs and genesencoding them may also be prepared by one skilled in the art asdescribed in U.S. Pat. No. 5,187,076 to Wozney et al.; U.S. Pat. No.5,366,875 to Wozney et al.; U.S. Pat. No. 4,877,864 to Wang et al.; U.S.Pat. No. 5,108,922 to Wang et al.; U.S. Pat. No. 5,116,738 to Wang etal.; U.S. Pat. No. 5,013,649 to Wang et al.; U.S. Pat. No. 5,106,748 toWozney et al.; and PCT Patent Nos. WO93/00432 to Wozney et al.;WO94/26893 to Celeste et al.; and WO94/26892 to Celeste et al. Allosteoinductive factors are contemplated whether obtained as above orisolated from bone. Methods for isolating bone morphogenetic proteinfrom bone are described, for example, in U.S. Pat. No. 4,294,753 toUrist and Urist et al., 81 PNAS 371, 1984.

[0073] The choice of carrier material for the osteogenic composition isbased on biocompatibility, biodegradability, mechanical properties andinterface properties as well as the structure of the load bearingmember. The particular application of the compositions of the inventionwill define the appropriate formulation. Potential carriers includecalcium sulphates, polylactic acids, polyanhydrides, collagen, calciumphosphates, hyaluronic acid, polymeric acrylic esters and demineralizedbone. The carrier may be any suitable carrier capable of delivering theproteins, nucleotide sequences, or the like. Most preferably, thecarrier is capable of being eventually resorbed into the body. Onepreferred carrier is an absorbable collagen sponge marketed by IntegraLifeSciences Corporation under the trade name Helistat® AbsorbableCollagen Hemostatic Agent. Another preferred carrier is a biphasiccalcium phosphate ceramic. Ceramic blocks and granules are commerciallyavailable from Sofamor Danek Group, B. P. 4-62180 Rang-du-Fliers, Franceand Bioland, 132 Rou d Espangne, 31100 Toulouse, France. Theosteoinductive factor is introduced into the carrier in any suitablemanner. For example, the carrier may be soaked in a solution containingthe factor.

[0074] In many cases, the osteoinductive factor may be included in thecalcium phosphate material prior to its hardening around the reinforcingmember to form the interbody fusion implant as the hardening typicallyis performed at or below 37° C. Alternatively, the factor, such as abone morphogenetic protein in a suitable liquid carrier, may be appliedonto and/or into the hardened, porous load bearing body after hardening,for instance by soaking, dripping, etc.

[0075] The interbody fusion implants of the invention may be providedwith surface features defined in their outer surfaces. In one form ofthe invention, for example, at least one of the ends of the implant is atool engagement end 27 that defines a tool engaging or instrumentattachment hole 28 as seen in FIGS. 3 and 4. In a preferred embodiment,hole 28 is threaded but any suitable attachment configuration iscontemplated.

[0076] Interbody fusion implants of the present invention may furtherinclude a tool-engaging slot 29 for receiving an implantation tool. Theslot is typically perpendicular to the central longitudinal axis AL ofthe implant, as shown, for example, in FIG. 3. In yet other embodiments,the slot 29 may serve as an alignment score mark or groove 29′ definedin tool engagement end 27′ of implant 10″ seen in FIG. 26, thus makingthe opposite end the insertion end. Implant 10″ is identical in allrespects to implant 10′, except for the difference in the featurepresent on an end of the implant and the absence of external threads.Thus, components of spacer 10″ are numbered correspondingly to those ofspacer 10′, except with a denoting prime “″” symbol.

[0077] Alternatively, a projection may be formed on the end wallsinstead of a slot. Such a projection may form a straight, flat-sidedshape (such as a mirror image of the slot depicted in FIG. 3), anelliptical eminence, a bi-concave eminence, a square eminence, or anyother protruding shape which provides sufficient end-cap or toolengaging end strength and drive purchase to allow transmission ofinsertional torque without breaking or otherwise damaging the eminence.

[0078] Yet other surface features can be defined along the length L ofthe spacer. As mentioned above with respect to FIGS. 3 and 4, the outersurface of the implant may define threads 190 or otherexpulsion-resistant configurations such as teeth, grooves, wafflepatterns, etc. The threads or other surface features may also stabilizethe bone-spacer interface and reduce micromotion to facilitate fusion.The implants of the present invention may be provided with threads bymethods well known to the skilled artisan such as incorporation ofthreaded features in a mold in which the load bearing body is hardened,and/or by machining the piece after hardening.

[0079] In certain embodiments, the threads or other expulsion-resistantsurface features may be formed from the reinforcing members, asillustrated in FIGS. 27 and 28. Implant 200 includes a load bearing body201 having disposed therein structural reinforcing member 90 whichincludes a body 91 which has a helical, or spiral, configuration. Loadbearing body 201 further has a first end 202, a second end 203, and awall 204 connecting first end 202 and second end 203. Wall 23 alsodefines a first, superior surface 205 and a second, inferior surface206. Reinforcing member 90 is disposed in load bearing body 201 so thatat least a portion of reinforcing member 90 is exposed from the outersurface of body 91 to form threads on outer surface 208 of implant 200.A substantial portion of reinforcing member 90 is embedded in loadbearing body 201 to provide fixation of the member 90 within body 201and preferably also to improve the resistance of implant 200 againstbending forces.

[0080] Referring now to FIGS. 29 and 30, an implant 250 is shown thatincludes plates that may provide reinforcement and further aid inpreventing expulsion of the implant after implantation. Implant 250includes a load bearing body 256 that has a first end 251, second end252 and a wall 253 connecting first end 251 and second end 252. Theillustrated implant includes an elongate reinforcing member 270 that isdisposed, and preferably partially embedded, in load bearing body 256.Reinforcing member 270 includes body 271, extending along the centrallongitudinal axis of implant 250. Plates 260 extend radially from body271 of reinforcing member 270, and are partially exposed on superiorsurface 254 and inferior surface 255 of implant 250 or are otherwisepartially embedded in load bearing body 256. The plates 260 may beconfigured and positioned to resist expulsion of the implant afterimplantation. Reinforcing member 270, with radially extending plates260, is best seen in FIG. 31.

[0081] In yet another aspect of the present invention, methods ofpromoting fusion bone growth between adjacent vertebrae are provided. Inone form of the invention, a method includes providing a first interbodyfusion implant described herein, such as one having a load bearing bodywith a reinforcing member disposed therein. The implant selected is ofthe appropriate dimensions, based on the size of the cavity created andthe needs of the particular patient undergoing the fusion. The adjacentvertebrae are prepared to receive the spacer in an intervertebral spacebetween adjacent vertebrae according to conventional procedures. Thespacer is mounted on an instrument known to the art, preferably via aninstrument attachment hole. An osteogenic material may optionally beplaced within a thru-hole, or gap, of the implant should one be present.The implant is then inserted into the cavity created between theadjacent vertebrae to be fused. Once the implant is properly orientedwithin the intervertebral space, the implant may be disengaged from theinstrument. In a preferred form of the invention, a second implant isinserted into the intervertebral space after the first implant isproperly positioned near vertebral body V, resulting in bilateralplacement of the spacers as seen in FIG. 32. Osteogenic material mayalso optionally be placed within those implants having thru-holes.

[0082] In a further aspect of the present invention, methods of makingan interbody fusion implant are provided. In one form of the invention,a method of making an interbody fusion implant includes providing a moldhaving positioned therein a structural reinforcing member. The mold willbe shaped as desired to form an implant having the desired shape. Thereinforcing member may include at least one of the reinforcing members,or similar members, described herein. A hardenable, flowable syntheticcalcium phosphate material, for example selected from materialsdescribed above, is then poured or otherwise passed into the mold. Thematerial is then caused to harden, by, for example, exposing thematerial to temperatures of 37° C. or below, and/or exposing thematerial to pressure.

[0083] While the invention has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character, it being understoodthat only the preferred embodiment has been shown and described and thatall changes and modifications that come within the spirit of theinvention are desired to be protected. In addition, all references citedherein are indicative of the level of skill in the art and are herebyincorporated by reference in their entirety.

What is claimed is:
 1. An interbody fusion implant, comprising: abiocompatible load bearing body, said body comprised of a syntheticcalcium phosphate material hardened around at least one structuralreinforcing member for resisting bending forces when implanted, saidbody sized and configured for engagement between two vertebrae andhaving a superior surface configured to contact one of said vertebrae,and an inferior surface configured to contact the other of saidvertebrae, said structural reinforcing member disposed between saidsuperior surface and said inferior surface and extending along a lengthof said body.
 2. The interbody fusion implant of claim 1, wherein saidcalcium phosphate ceramic is a calcium phosphate apatite.
 3. Theinterbody fusion implant of claim 2, wherein said calcium phosphateapatite is a low crystallinity apatite.
 4. The interbody fusion implantof claim 1, wherein said reinforcement member is comprised of a metal.5. The interbody fusion implant of claim 4, wherein said metal istitanium.
 6. The interbody fusion implant of claim 5, wherein saidtitanium is a titanium mesh.
 7. The interbody fusion implant of claim 5,wherein said metal is selected from the group consisting of titanium,stainless steel, cobalt-chromium, tantalum, mixtures thereof and alloysthereof.
 8. The interbody fusion implant of claim 1, wherein saidimplant has a compressive strength of at least about 40 MPa.
 9. Theinterbody fusion implant of claim 1, wherein said body further comprisesa tool engaging end defining a tool engaging hole for receiving adriving tool for implanting the spacer.
 10. The interbody fusion implantof claim 1, wherein said body has an outer surface that defines threadedbone-engaging portions.
 11. The interbody fusion implant of claim 1,wherein said implant is a dowel.
 12. The interbody fusion implant ofclaim 1, wherein said implant is a wedge.
 13. The interbody fusionimplant of claim 1, wherein said body further includes a wall connectingsaid superior surface and said inferior surface.
 14. The interbodyfusion implant of claim 13, wherein said body is elliptical.
 15. Theinterbody fusion implant of claim 1, wherein said body further definesat least one thru-hole.
 16. The interbody fusion implant of claim 15,wherein said body has a longitudinal axis and said thru-hole extendsperpendicular to said longitudinal axis.
 17. The interbody fusionimplant of claim 16, wherein said body further includes an osteogenicmaterial disposed within said thru-hole.
 18. The interbody fusionimplant of claim 17, wherein said osteogenic material comprises naturalbone, demineralized bone, a calcium phosphate material, a bioceramic,bioglass, an osteoinductive factor and mixtures thereof.
 19. Theinterbody fusion implant of claim 18, wherein said osteoinductive factorcomprises a bone morphogenetic protein.
 20. The interbody fusion implantof claim 19, wherein said bone morphogenetic protein comprises arecombinant protein.
 21. The interbody fusion implant of claim 20,wherein said recombinant bone morphogenetic protein comprises a humanprotein.
 22. The interbody fusion implant of claim 21, wherein saidrecombinant human protein comprises BMP-2, BMP-4, BMP-7, or heterodimersthereof.
 23. The interbody fusion implant of claim 1, wherein saidreinforcing member extends parallel to said superior and inferiorsurfaces.
 24. An interbody fusion implant, comprising: a) abiocompatible load bearing body, said body comprised of a hardenedsynthetic calcium phosphate material, said body sized and configured forengagement between two vertebrae and having a superior surface and aninferior surface; and b) at least one structural reinforcing member forresisting expulsion after implantation, said structural reinforcingmember at least partially embedded in said load bearing body andconfigured to contact adjacent vertebrae.
 25. An interbody fusionimplant, comprising: a load bearing body formed of a hardened syntheticcalcium phosphate material, said body containing at least one internalreinforcing member adapted to resist bending or tensile forces along alength of said body, said body sized and configured for engagementbetween two vertebrae and having a first surface for contacting a firstof said vertebrae and a second surface for contacting another of saidvertebrae.
 26. A method of promoting fusion bone growth between adjacentvertebrae, comprising: (a) providing an interbody fusion implantcomprising: (i) a porous, biocompatible load bearing body, said bodycomposed of a synthetic calcium phosphate material hardened around atleast one structural reinforcing member for resisting bending forceswhen implanted, said body sized and configured for engagement betweentwo vertebrae and having a superior surface configured to contact one ofsaid vertebrae, and an inferior surface configured to contact the otherof said vertebrae, said structural reinforcing member disposed betweensaid superior surface and said inferior surface and extending along alength of said body. (b) preparing said adjacent vertebrae to receivethe implant in an intervertebral space between adjacent vertebrae; and(c) placing the implant into the intervertebral space.
 27. The method ofclaim 26, wherein said body defines a thru-hole extending therethrough.28. The method of claim 27, further comprising filling said thru-holewith an osteogenic material prior to said placing the implant into theintervertebral space.
 29. A method of making an interbody fusionimplant, said method comprising: (a) providing a mold having positionedtherein a structural reinforcing member; (b) passing a hardenablesynthetic calcium phosphate material into the mold; and (c) causing saidmaterial to harden to form a load bearing interbody fusion implant, saidimplant sized and configured for engagement between two vertebrae andhaving a superior surface configured to contact one of said vertebrae,and an inferior surface configured to contact the other of saidvertebrae.
 30. The method of claim 29, wherein said implant is a dowel.31. The method of claim 29, wherein said implant is a wedge.